1. Field of the Invention
The present invention relates to an optical modulator which is suitable for use in a high-speed, broadband optical fiber communication system, a method of achieving velocity matching and impedance matching of such an optical modulator, and a method of manufacturing such an optical modulator.
2. Description of the Related Art
In recent years, with the progress of high-speed, broadband optical fiber communication systems, high-speed optical modulators using optical waveguide devices such as external modulators have been widely used in practical applications. Since such an optical modulator modulates a lightwave guided through an optical waveguide with a modulating signal in a microwave range, for example, it is important for the optical modulator to achieve velocity matching and impedance matching between the lightwave and the modulating signal.
In an attempt to achieve velocity matching and impedance matching, Japanese laid-open patent publication No. 9-211402 discloses an optical modulation device.
According to the disclosure, a supporting substrate is mounted on the reverse surface of a substrate of the optical modulation device. The supporting substrate has a cavity at a position corresponding to a modulating region of the optical modulation device for lowering the effective refractive index of a modulating signal thereby to suppress a reduction in the phase velocity of the modulating signal. Suppressing a reduction in the phase velocity of the modulating signal is effective to achieve velocity matching between a lightwave guided through an optical waveguide of the optical modulation device and the modulating signal that is applied from a modulating electrode of the optical modulation device, thus achieving impedance matching.
According to the above arrangement, the substrate of the optical modulation device needs to be as thin as several μm for achieving sufficient velocity matching and impedance matching. However, it requires a high level of machining accuracy to produce the thin substrate, and hence it has been difficult to manufacture an optical modulator incorporating the above optical modulation device. The supporting substrate with the cavity is liable to suffer stresses concentrated around the cavity, tending to cause a DC drift and a thermal drift which make it difficult for the optical modulator to operate stably. The stress concentration may destroy the optical modulation device, i.e., the optical modulator undesirably.